The present invention relates to a tube having a welded bottom, particularly for making a high-pressure gas or pneumatic spring, and to a process for the manufacture thereof.
Pneumatic springs, which are used for example for balancing tailgates of motor vehicles, are known. The good functioning of these springs requires that they should be filled with a gas under a very high internal pressure, which may be as high as 150 bars and sometimes even higher. In order to eliminate all risk of leakage or explosion of the welded-end tube which is the essential component of a pneumatic spring of this kind, it is necessary that the weld joining an end piece intend to form an end face of the tube should be of exceptional quality.
Up to the present time the end piece, consisting of a metal disk whose transverse dimensions are greater than the inside transverse dimensions of the tube, was applied from outside to the open end of the tube, to which the disk was then welded, likewise from outside the tube. The considerable pressure forces applied to the the welded end of a tube of this kind, which is used to make a pneumatic spring, tend to separate the end from the tube, and these forces are taken solely by the weld. It has been found that this process did not enable the risk of explosion to be completely eliminated, for the following reasons: on the one hand, the metal disk intended for closing the tube is sometimes taken from a piece of steel of poor quality, which should have been scrapped but which has by mistake been mixed with pieces of higher quality; and on the other hand, the operation of the welding machine is subject to fluctuations, due in particular to microcuts, brief voltage drops, and so on, which are detrimental to the definitive quality of the weld.
The welded-end tube according to the present invention has on the contrary, particularly at its welded end, strength properties which enable it to be used in particular for the production of a high-pressure pneumatic spring practically free from all risk of leakage or explosion, without it being indispensable to use a very high grade steel for making the end piece of the tube.
British Pat. No. 376,957 issued on July 21, 1932, to Mannesmann-Rohren-Werke et al. discloses a welded-end tube wherein the edge of a first open end of the tube is folded over towards the interior of the tube and an end piece, whose transverse dimensions are between those of the opening bounded by the folded-over edge of the tube and the inside transverse dimensions of the tube, is introduced into the tube and welded to its folded-over edge.
As the result of this arrangement, when the welded-end tube is used for making a high-pressure pneumatic spring the powerful pressure forces which act on its end are not taken solely by the weld but also, and perhaps mainly, by the folded-over edge of the tube, against which the end piece forming its end is applied from the interior of the tube. Moreover, the region of the end piece which is welded to the folded-over edge of the tube is protected by this folded-over edge, which constitutes a kind of protective shield protecting the weld against shocks and above all against corrosion. The external appearance of the end of the tube is also improved, because the welded end has no sharp edge and, if the folded-over edge of the tube has any burrs, these are far less visible.
However, in this arrangement, the welded zones of the tube have a large surface exposed to the external atmosphere, resulting in a high likelihood of corrosion. For this reason, the life of the welded-end tube is substantially reduced, especially if the tube is used in exterior environments. The choice of suitable metals for the tube therefore is also limited.
An object of the present invention is to provide a welded tube of the above-mentioned type, having an improved protection against corrosion.
Another object of the invention is to provide a pneumatic spring comprising a welded tube of the above-mentioned type, having an improved duration of life when used in exterior environments.
According to the invention, there is provided a tube having a welded bottom impermeable to gases under high pressure, the tube comprising (a) a tube body having an end with an inner peripheral edge folded-over toward the interior of the tube body for forming an opening of reduced diameter; and (b) an end piece for forming the welded bottom of the tube, the end piece being a metal disk with an outer diameter substantially smaller than the inner diameter of the tube body and substantially greater than the reduced diameter of the opening of the inner peripheral folded-over edge of the tube end, the disk being provided at the center thereof with a cylindrical outwardly directed projection, the projection having a diameter slightly smaller than the reduced diameter of the opening of the peripheral folded-over edge of the tube end, wherein the end piece is located inside the tube body and joined to the folded-over edge by a welded zone having a metallographical structure of melted or softened metal, the welding zone having an exposed surface in the bottom of a narrow groove formed between the folded-over edge and the projection.
According to an aspect of the invention, the projection supports means, such as a threaded rod or an eye-piece for connecting the tube to another mechanical member, e.g. a support.
According to another aspect of the invention, the projection has a height such that the end face of the tube formed by the end piece and tube has a substantially smooth outer surface.
According to one of the significant features of the present invention, the end piece is provided, at its center, with a cylindrical projection having a diameter which is just slightly smaller than the diameter of the opening bounded by the folded-over edge of the tube. Consequently, the end face of the tube forms a circular narrow groove limited by the folded-ver tube and by the projection. The exposed surface of the welded zone occupies only the bottom of this groove. For this reason, the welded zone is protected against corrosion agents. It can easily be completely protected, for instance, by filling the groove with a protective substance.
A process for the production of a welded-end tube according to the present invention comprises folding over the edge of a first open end of the tube towards the interior of the latter by a cold-forming process such as crimping, rolling, beading or a similar process, engaging the end piece inside the tube by way of its second open end in such a manner as to bring the end piece into contact with the folded-over edge of the tube, and welding the folded-over edge to the end piece in contact with it.
As the pressure forces applied to the welded end of the tube constituting a pneumatic spring are not taken exclusively by the weld, and the latter is only a little or not at all exposed to corrosion, it may be of lower quality, that is to say any slight defects in it are less likely to lead to leaks or even an explosion of the pneumatic spring. For the production of both the tube and its end piece it is therefore possible to select lower grades of weldable steel, which are thus less expensive; for the production of the end piece it is also possible to select a steel better suited to the method of production of this end piece.